In a storage control apparatus, a storage unit is provided in which a large number of hard disk drives are connected in the form of an array, and such a device supplies a logical storage region (a logical volume) to a host computer such as a server or the like (hereinafter termed a “host”). In order to enhance the reliability and so on, the storage region supplied to the host by the storage control apparatus is made redundant on the basis of RAID (Redundant Array of Independent Disks).
The amount of data which must be managed by a business or the like increases day by day. Accordingly it is desirable for a storage control apparatus to be able to increase its storage capacity in correspondence to this increase of the amount of data. Thus, a storage control apparatus is per se known (for example, refer to Patent Citation 1) which can supply a storage region expanded according to the wish of the user, by a desired number of additional chassis containing additional storage units being added on to the basic chassis containing the basic storage units.
With this type of storage control apparatus, in order to economize upon electrical power, it is per se known to performing turning ON and OFF of the power supply to each of the additional chassis (for example, refer to Patent Citation 2).
Now, with a storage control apparatus to which a plurality of storage devices (for example a plurality of hard disk drives) are mounted and to which the RAID technique is applied, if a failure should take place with any one of the plurality of data storage devices which makes up some RAID group, then, as a method of procedure for recovering from this breakdown using a spare storage device, in the prior art, either of the methods (1) and (2) described below has been used.
(1) Copy-Back
Data which is the same as the data stored on the faulty storage device is recovered upon the spare storage device. After the faulty storage device has been exchanged for a new storage device, the recovered data upon the spare storage device is copied back to the new storage device. And the new storage device is installed to the RAID group in question as a new data storage device. In this specification, this method of procedure will be termed “copy-back”.
(2) Copy-Backless
Data which is the same as the data stored on the faulty storage device is recovered upon the spare storage device. And this spare storage device is installed to the RAID group in question as a new storage device. A new storage device, which has been exchanged for the faulty storage device, is now used as a new spare storage device for this RAID group. In this specification, this method of procedure will be termed “copy-backless”.
The merit of the copy-backless procedure, as compared with the copy-back procedure, is that the time period required to recover from a fault is shortened. In the prior art, the user has been able to determine at will at which positions the plurality of storage devices and spare storage devices which make up each RAID group are fitted in the plurality of additional chassis, and which of copy-back or copy-backless operation is selected.    [Patent Citation 1]    Japanese Laid-Open Patent Publication 2001-339853;    [Patent Citation 2]    Japanese Laid-Open Patent Publication 2008-090352.